Process for the treatment of carburizing salt wastes

ABSTRACT

This invention relates to a process for the treatment of the reaction mass produced during the joint elimination of barium-containing nitrite-/nitrate-bearing carburizing salt wastes and cyanide-/cyanate-bearing wastes by ignition of the dry mixture at elevated temperatures, comprising suspending the reaction mass in water and either 
     A. neutralizing the alkaline suspension obtained with concentrated nitric acid to a pH value of 6, the feed velocity of the acid and conditions of agitation being so selected that the pH value of the suspension will at no time fall below pH 6, and separating the barium carbonate, 
     Or 
     B. freeing the alkaline suspension obtained from barium carbonate, and neutralizing the filtrate with concentrated nitric acid to a pH value of 7.2, the feed velocity of the acid and conditions of agitation being so selected that the pH value of the suspension will at no time fall below pH 7.2, and separating the precipitated sodium bicarbonate, 
     Concentrating the clear solutions obtained according to (a) or (b) to a water content of 10 - 20% by weight, separating precipitated sodium chloride, and evaporating the residual solution to dryness.

The present invention relates to a process for the treatment of thereaction mass which is produced during the joint elimination ofbarium-containing nitrite-/nitrate-bearing carburizing salt wastes andcyanide-/cyanate-bearing wastes by ignition of the dry mixture atelevated temperatures.

Known in the art from German Pat. No. 2,234,171, is a process for thejoint elimination and/or treatment of possibly barium-containingcarburizing salt wastes based on nitrite-/nitrate-bearing wastes, on theone hand, and cyanide-/cyanate-bearing wastes, on the other hand, byoxidation of the cyanide/cyanate by nitrite/nitrate. In this processoxidation of the cyanide-/cyanate-bearing constituent of the carburizingsalt wastes with the nitrite-/nitrate-bearing constituent of thecarburizing salt wastes takes place at an excess by ignition of the drymixture at temperatures above 150°C, and the resulting bariumcarbonate-containing mixture is then leached out with water.

If, according to the process of this patent, an elimination rather thana treatment of the barium salt-containing carburizing salt residues issought, one adds, prior to the thermal reaction, an amount -corresponding to the barium constituent - of a sulfate of calcium,sodium, or iron. After the thermal reaction, a brittle reaction mass hasformed which can be discarded as a non-toxic, neutral salt-containingrefuse, (main constituent barium sulfate). If, in the process, one workswithout sulfate addition, the reaction mass contains barium carbonate asthe main constituent. When this mixture is leached out with water, thesoluble constituents, such as sodium carbonate, potassium chloride andsodium chloride, are dissolved. After filtration of the barium carbonatesludge, the strongly alkaline filtrate obtained is disposed of as wastewater, after neutralization with sulfuric acid.

It now has been found in the meantime in actual practice that, dependingupon the mode of the process (with or without addition of sulfate),either the waste water - because of the dissolved salts when proceedingwithout sulfate addition, or the environment - because of thecorresponding special refuse load (after addition of sulfate) areaffected to a significant degree.

In order to now obtain re-usable salts, after the thermal reaction ofthe cyanide-/cyanate-bearing mixture with the nitrite-/nitrate-bearingmixture (which was carried out without sulfate addition in order that noinsoluble waste products and/or end products be obtained, and in orderto considerably reduce the salt content of the waste water), oneproceeds in such a manner, according to the present invention, that thereaction mass is suspended in water, and the alkaline suspensionobtained is

a. either neutralized with concentrated nitric acid to a pH value of 6,whereby the flow velocity of the acid and the stirring conditions are sochosen that the pH value of the suspension will not fall below pH 6, andthe suspension is then filtered in order to separate the bariumcarbonate,

b. or freed from barium carbonate by filtration, and the filtrate isneutralized with concentrated nitric acid to a pH value of 7.2, wherebythe flow velocity of the acid and the stirring conditions are so chosenthat the pH value of the suspension will not fall below pH 7.2, and theprecipitated sodium bicarbonate is thereafter separated, whereupon theclear solutions obtained according to (a) or (b) are concentated to awater content of 10 - 20% by weight; the sodium chloride precipitatedthereby is separated, preferably at a temperature in the range of 70° -100°C, and the residual solution is evaporated to dryness, at which timea directly reusable tempering salt is produced.

In the inventive process, the cyanide-/cyanate-free reaction mass isthus leached out with water after the thermal reaction has been carriedout. Obtained thereby is a sludge which contains, as an undissolved mainconstituent, barium carbonate as well as, as a dissolved constituent,mainly sodium chloride, alkali carbonate, and alkali-nitrite/nitrate. Ina neutralization reactor which is equipped with a high-speed turbomixerand a metering apparatus for the nitric acid, the strongly alkalinefinal liquor is then neutralized with concentrated nitric acid to a pHvalue of 6. The nitrate formed thereby thus enables one to eliminate analkali nitrate addition otherwise necessary for obtaining a low-meltingpoint tempering salt end product.

Of great importance in this process step is, on the one hand, themetering velocity of the nitric acid and, on the other hand, the rapidintermixing of the reactants by vigorous stirring. A localover-acidification is thereby prevented, and the formation of nitrousgases is thus counteracted. After the completed neutralization, theundissolved portion (mainly barium carbonate) is separated from the saltsolution by decantation and filtration. The washed reaction residue(barium carbonate) is well suited for the preparation of bariumchloride.

It is also possible, however, according to the present invention, toproceed in a manner such that, prior to the neutralization with thenitric acid, the suspension is freed from insoluble barium carbonate andthe clear solution is then neutralized with the nitric acid to a pHvalue of 7.2. The reaction conditions are the same as have beenindicated in the preceding paragraph. The sodium bicarbonate whichprecipitates at the pH value of 7.2 is filtered off.

The recovery of the sodium bicarbonate precipitated as a result of thisreaction is carried out in order that one does not have to convert theentire alkali carbonate constituent into non-usable carbon dioxide, andin order to form less nitrate than is the case under (a) above, if suchshould be necessary in view of the NO₃ -NO₂ -equilibrium in thetempering salt reclaimed product. Instead, the precipitated sodiumbicarbonate may be further utilized economically.

The chloride-containing nitrite-/nitrate filtrate obtained according to(a) or (b) contains 50 to 60% by weight of water and is thenconcentrated for the purpose of separating the chloride constituent in acrystallizing evaporator to a water content in the range of 10 to 20% byweight. The concentrate containing less than 1.5% by weight of chlorideis thereafter evaporated, in a thin-layer evaporator, to a water-freemelt which solidifies on a cooling bath and is scaled off.

A further advantage resides in the fact that the chloride portionseparated only as sodium chloride influences equally favorably thesodium-potassium equilibrium in the tempering salt reclaimed productsince the entire wastes contain relatively more sodium than is desirablein the low-melting tempering salt.

When this inventive process was actually carried out, it was found thatanother process step of the above-mentioned patent also may be deleted.namely that in which the nitrite-/nitrate-bearing residues are melteddown, and a main portion of this nitrite-/nitrate-bearing melt isseparated by filtration and must be subjected to a separate treatment.As a result thereof, it was effectively prevented that too much excessnitrite/nitrate was fed to the thermal oxidation reaction which had tobe reduced by additions of coke.

This step is now no longer necessary since the excess nitrite/nitrate isobtained in reusable form, according to the novel process mode ofoperation, and in the wet state at the end of the inventive process,together with the nitrate additionally produced due to theneutralization reaction. This mode of operation is enhanced by the factthat during the detoxification reaction with a nitrite-/nitrate excess,nitrate is preferentially consumed.

The alkali-nitrite/nitrates obtained can at once be employed again asusable tempering salts in salt hardening rooms.

The inventive process will now be further described hereinafter on thebasis of the followng examples:

EXAMPLE 1

9.65 parts by weight of cyanide-/cyanate-free reaction product from thedetoxification reaction carried out between cyanide-/cyanate-bearinghardening room salt wastes and nitrite-/nitrate-bearing hardening roomsalt wastes are introduced, still hot, into 10 parts by weight of waterwhile stirring. Obtained thereby is a sludge which contains 2.9 parts byweight of barium carbonate and 0.4 part by weight of other undissolvedsubstances, as well as 2.7 parts by weight of sodium chloride, 1.4 partsby weight of alkali carbonate, and 2.25 parts by weight of dissolvedalkali-nitrite/nitrate.

Introduced into this suspension at 50°C and while stirring vigorouslywith a turbomixer during 3 hours are 2.2 parts by weight of concentratednitric acid. At that time, the nitric acid is metered into the suctionzone of the stirrer in a fine spray in order to avoid anyover-acidification. The pH value of the solution changes thereby from aninitial 9.5 to 6.0. During the entire neutralization operation, no rednitric oxide vapors are noticeable. After the neutralization, theundissolved portion (main constituent barium carbonate) flocculates outrapidly and is separated from the salt solution by decanting andfiltering.

The clear chloride-containing nitrite-/nitrate solution obtainedcontains approximately 60% by weight of water and, for purposes ofseparation of the chloride, is concentrated in a vacuum evaporator at90°C and 250 Torr to 14% by weight of water. At that time, the mainamount of the chloride crystallizes out as sodium chloride, whereas thealkali-nitrite/nitrate remains completely dissolved at 90°C and isseparated by centrifuging. The sodium chloride which is rewashed withsaturated sodium chloride and a little water contains thereafter stillless than 0.01% nitrogen from NO₂ -- and NO₃ -- as well as more than 98%by weight of sodium chloride.

The nitrite-/nitrate-bearing solids are stirred to complete waterremoval under vacuum at 80° to 90°C. Obtained thereby ultimately are4.30 parts by weight of crystalline water-free alkali-nitrite/nitratepowder containing 1.4% by weight chloride. This product forms at 160°C aclear melt and is thus directly usable as tempering salt.

EXAMPLE 2

8.4 parts by weight of cyanide-/cyanate-free reaction product from thedetoxification reaction carried out between cyanide-/cyanate-bearinghardening room salt wastes and nitrite-/nitrate-bearing hardening roomsalt wastes are treated with 8 parts by weight of water as in Example 1.The sludge contains 2.4 parts by weight barium carbonate, 0.35 part byweight of other insoluble substances, 2.1 parts by weight of dissolvedsodium chloride, 1.2 parts by weight of alkali carbonate, partiallydissolved, and 2.35 parts by weight of dissolved alkali-nitrite/nitrate.The undissolved part which contains the entire quantity of bariumcarbonate is separated by decantation, filtration and washing.

The alkaline washing water is combined with the filtrate which is nowreacted while undergoing an intensive thorough blending with 1.0 part byweight of concentrated nitric acid to a pH value of 7.2 for purposes ofthe conversion of the entire alkali carbonate amount to the bicarbonate.After the separation of the relatively insoluble sodium bicarbonate,precipitated in an amount of 1.7 parts by weight, there follows aconcentration of the solution to a water content of 15% by weight.Evaporation takes place under vacuum so that the boiling point will beat a temperature in the range of 90° to 100°C, which suffices tomaintain all the nitrite and nitrate in solution. After the completedsalting-out, the salt paste is centrifuged and the wet solids furtherevaporated under vacuum as described in Example 1. Obtained in thismanner are 2.75 parts by weight of an alkali-nitrite/nitrate salt,containing 1.4% by weight of chlorine, which forms a clear melt at160°C.

It will be obvious to those skilled in the art that many modificationsmay be made within the scope of the present invention without departingfrom the spirit thereof, and the invention includes all suchmodifications.

We claim:
 1. A process for the treatment of the reaction mass producedby igniting a dry mixture of barium-containing nitrite-/nitrate-bearingcarburizing salt wastes and cyanide-/cyanate-bearing wastes attemperatures above about 150°C,comprising suspending the reaction massin water to obtain a suspension containing sodium chloride, alkalicarbonate, and alkali nitrite/nitrate as dissolved constituents, andbarium carbonate as an undissolved constituent, neutralizizng thealkaline suspension obtained with concentrated nitric acid to a pH valueof 6, the feed velocity of the acid and conditions of agitation being soselected that the pH value of the suspension will at no time fall belowpH 6, separating the flocculated barium carbonate, concentrating theclear solution obtained to a water content of 10 - °% by weight toprecipitate sodium chloride, separating the sodium chloride, andevaporating the residual solution to dryness.
 2. A process according toclaim 1 in which the precipitated sodium chloride is separated at atemperature in the range of about 70° - 100°C.